Radio frequency wattmeter



thermo-couple heater unit Ri,

Patented Mar. 2l, '1944 RADIO FREQUENCY WATTMETER Charles A. Rosencrans, Haddoniield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 2, 1942, Serial No. 464,299

14 Claims.

This invention relates generally to electrical measuring instruments and particularly to wattmeters for measuring the radio frequency power y applied to high frequency transmission lines.

Various methods and apparatus utilized heretofore for the measurement of power in radio frequency transmission lines have involved the use of relatively complicated circuits which, in many instances, have necessitated the absorption of considerable power from the transmission line. The instant invention contemplates the use of a simple coupling unit which provides inductive and capacitive coupling to one or more conductors of the transmission line. The coupling unit is connected at its extremities through two thermo-couple heater elements to a suitable return circuit such as ground. The device operates upon the principle of indicating the diierence between the squares of the sum and the difference of the currents derived from the inductive and capacitive coupling, respectively, to the line. For most satisfactory operation, the coupling unit should be loosely coupled to the line to provide a relatively low series inductance in the line, and a relatively low The sensitivity of the indicating instrument may be adjusted by varying the shunt capacity to the line. However, any increase in capacity will increase the meter load upon the line. In all cases. the shunt resistance on the line provided by the thermo-couple heater elements should be considerably less than the shunt reactance of the capacity coupling and should exceed the series reactance of the series inductance in the line.

Under these conditions line input power to the coupling unit will be proportional to the line volt-v age El and line current Il. Since the wattmeter is loosely coupled to the line and reflects arelatively *low series reactance into the line, thel output voltage and current E2 and I2 will likewise be substantially proportional to theV input voltage and current El, I I. The inductive coupling, represented by the mutual inductance m, which should preferably be unitary, will provide an inductive current Il in the loop circuit comprising the inductive coupling unit, the first the return circuit and the second thermo-couple heater unit R2. Therefore, the inductive'current Il will be in dii'- ferent directions in the two thermo-couple heatv er units RI, R2.

The capacitive current Ic in the loop circuit. however, will ilow in the same direction through both thermo-couple heater units to the grounded return circuit. Therefore, the inductive and cashunt capacity to the line.,

.ing element coaxial with,

pacitive currents will have an additive eil'ect upon the first thermo-couple heater unit RI, and a differential effect upon the second thermo-couple heater unit R2. A meter connected in series with both thermo-couples will therefore indicate the difference of the squares of the sum and difference of the inductive and capacitive currents to the thermo-couple heater elements. Since the inductive current is proportional to the transmission line current at a. phase angle of and the c-apacitive current is proportional to the transmission line voltage at a, phase angle of minus 90, the meter indication will be proportional to the power in the transmission line.

Among the objects of the invention are to provide a new and improved method of and means for measuring power in a radio frequency transmission linc. Another object of the invention is to provide a new and improved method of and means for measuring the power in a radio frequency transmission line wherein the measuring instrument absorbs a minimum amount of power from the line and reiiects minimum reactance ther ein. A further object of the invention is to provide an improved method of and means for measuring power in a radio frequency transmission line which comprises a cylindrical conductportion of one or more of the conductors of the transmission line, thermo-couple means connected adjacent the extremities of the coupling unit. and a return circuit to form an external loop circuit. l

A further object of the invention is to provide an improved method of and means for measuring the power in a. balanced radio frequency transmission line wherein coaxial, cylindrical coupling devices are vprovided for each conductor of the line, and thermo-couple elements are connected between adjacent extremities of the conductive coupling units. Another object is to provide a power measuring device for a coaxial transmission line wherein a cylindrical conducting coupling unit surrounds the inner conductor of the transmission line, thermo-couple units are connected from the extremities of the coupling unit to adjacent portions of the outer conductor of the transmission line, and means for connecting the thermo-coupled units to a shielded external meter circuit.

The invention will be described by reference to the accompanying drawing oi' which Figure 1 is and surrounding, a

diagram of a second 1- constructionaldetails of the coupling unit for a mission line,

shown in -dashed lines.

ducting element v couple heater element R2 in the embodiment Ashowing 'the balanced transmission line; and Figure is a cross-sectional diagram of an embodiment of the invention adapted to use with a coaxial trans- Similar reference numerals are applied to similar elements throughoutthe drawing.

Referring to Figure 1, a` transmission line including the conductor l and the grounded return circuit 2 is supplied withradio frequency'power from a source, not shown, and coupled to a load circuit. not shown. The linevoltage is represented byEl and the line current by Il. The coupling unit, which will be described in greater detail hereinafter, comprises an inductive element LI ln series with tively coupled to a second inductive element L2. The inductive elementsI Ll and L2should have substantiallyunitary mutual inductance. The

coupling unit also includes'capacity coupling between the transmission line l and the coupling coil L2 which is represented by the capacitors C One terminal A o! `the inductive coupling coil L2 is connected through the heater element'Rl of a rst thermo-couple 3 to the return circuit 2 of the transmission line., The remaining terminal of the'coupling eoi.l L2 is connected through the heater element R2 o! a of the transmission line. The thermcouples l and l are connected in series through a conven-- schematic circuit described in 4Figure 1. The

transmission line conductor l is surrounded for a portion o! its length by a coaxial cylindrical con- L which should preferably be considerably less than 1/2 wave length long. One extremity A. of thecoupling unit L is connected through the heater element Ri of the rst thermo-couple 3 to the grounded return circuit 2 at the point C. The other extremity B of the coupling unit L is connected'through the heater element R2 of the second thermo-couple 4 to the point D of the return circuit 2. The thermocouples 3, 4 are connected in series to the terminals of a microammeter M in such a manner that the meter indicates the difference between the currents generated in the two thermo-couples. A variable capacitor CI may be connected between the and the midpoint of the coupling device L. Preferably, the connection to the transmission line I should be made adjacent the connection to the coupling device L. Y A

In operation, the coupling device L represents both inductive and capacitive --coupling to the transmission line conductor `l. The capacitive coupling to the line may be increased by the variable capacitor Cl to increase the sensitivity of the indicating instrument, but'as pointed out y gassosi ,..showlngz.constnlctionalgdetails-cof the ,xic'mxiiing .Y unit; Figure lis va,partially.'schemiaizic circuittosnbstantially equal potential L .between the e`x the transmission line l induc ductive and capacitive current flow for any in-y stant, the inductive and capacitive currents Il and Ic. respectively, will have an additive eilect upon the ilrst thermo-couple heater element Rl and a dierential effect upon the second thermo-couple heaterfelement R2.'n Since normal thermocouple action provides thermo-couple currents proportional to the squares of the currents in the respective heater elements, the indication provided by the microammeter M will be proportional to the diilerence betweenV the squares of the sum and diierence o! the inductive and capacitive currents, namely Figure 3 is a modlcation of the use of the coupling unit L and the thermo-couples 3, 5 for a balanced transmission line wherein separate cousecond thermo-couple 4 to the return circuit 2 line. The first thermo-couple is connected between adjacentextremities of the -pling units La and Lb are utilized for each of the conductors Vi Vand ii of the balanced transmission heater element Ri two coupling units La and Lb whilethe heater velement R2 of the, second thermo-couple 4 is connected between. the other' adjacent extremities of the two coupling units La, Lb.

AFigure 41s a modification-of the circuits cf Figures 1 and 2 in which the invention is adapted to use with acoaxial transmission line compris- 'i ing an inner conductor 2| and an outer shield conductor l of the transmission line heretofore, an increase in the capacitive coupling l will increase the power absorbed from the transcated by the arrows, namely through points A, C, D, B inthe order named. The capacitive currents generated in the coupling unit L, however, will ilow directly from the manner indipoint A t0 the pontl'l 22. A coupling unit L is supported in any con' venient manner coaxial with, and surrounding, the inner conductor 2| of the coaxial line. The rst thermo-couple heater element Rl is connected between the outer shield 22 and one extremity of the coupling unit L. The second thermo-couple heater element R2 is connected between theouter shield 22 and the other extremity of the coupling unit L. The thermocouples are connected in series to the microammeter M in the same manner as described heretofore tor the other modications of the invention. However, M should be shielded, as indicated by the dash lines 5, to prevent radiation or pickup from external sources.

It should be understood that an auxiliary capacity CI may be employed with this modification in the same manner as that described in Figure 2, to increase the sensitivity of the indicating instrument; However, as mentioned heretofore, an increase in the capacitive coupling between the coupling unit Land the inner conductor 2 i results in increased absorption by the measuring circuit.

Thus the invention described comprises animproved radio frequency wattmeter which employs unitary inductive coupling and relatively small capacitive coupling to one or more conductors of a radio frequency transmission line. Inductive and capacitive currents owing through the couplings are applied to two thermocouples to pro-- the leads to the microammetei' .and capacitive coupling capacitive coupling for deriving substantially inpnase currents proportional to the current and the voltage respectively in said line, meansV includi said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means for applying said in-phase vcurrents to one of said current-responsive devices, means for applying said oppositely phased currents to the other of said current-responsive devices, an indicator, and means connecting said indicator to said current-responsive devices. Y

2. A radio frequency wattmeter for a power transmission line including a coupling device comprising unitary means surrounding and coaxial with a portion o1' one of the conductors oi said line for both inductively and capacitively coupling said device to said line, a pair f current-responsive devices, means including said inductive and capacitive coupling for deriving substantially inphase currents proportional to the current and the voltage respectively in said line, means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means for applying said in-phase currents to one of said currentfresponsive devices, means for applying said oppositely phased currents to the other oi said current-responsive devices, and means connected to said current-responsive devices to indicate the difference of the squares oi' the currents in each of said current-responsive devices.

3. A radio frequency 'wattmeter for a power transmission line including a coupling device comprising a cylindrical conducting member surrounding and substantially coaxial with av portion of said line for inductively and capacitively coupling said device to said line, ,a pair of currentresponsive devices, means including said inductive and capacitive coupling for deriving substantially in-phase currents proportional to the current and thevoltage respectively in said line,l means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means for applying said in-phase currents to one of said current-responsive devices, means for-applyingsaid oppositely phased currents to the other of said current-responsive devices, an indicator, and means connecting said indicator to said currentresponsive devices.

4. A radio frequency wattmeter fora power transmission line including a coupling device comprising a cylindrical conducting member surrounding and substantially coaxial with a portion of said lineV for inductively and capacitively coupling said device to said line, a pair of currentresponsive devices, means including said inductive in-phase currents proportional to the current and the voltage respectively in said line, means including` said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means for applying said in-phase currents to one of said current-responsive devices, means for applying said oppositely phased currents to the other of said current-responsive devices, and means connected to said current-responsivedevices to indicate the dii'- ierence oi' the squares ot the currents in each of said current-responsive devices.

. said line, a pair of Arespectively in said line,

5. A radio frequency wattmeter for a power transmission line including a coupling device comprising a cylindrical conducting member surrounding and substantially coaxial with a portion of one of the conductors of said line for inductively and capacitively coupling said device to said line, a pair of current-responsive devices. means including said inductive and capacitive coupling for deriving substantially in-phase currents proportional to the current and the voltage means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means lor applying said in-phase currents to one of said current-responsive devices, means for applying said oppositely phased currents to the other of said current-responsive devices, an indicator, and means connecting said indicator to said current-responsive devices.

6. A radio frequency wattmeter for a power transmission line including a coupling device comprising a cylindrical conducting member surrounding and substantially coaxial with a portion oi' one of the conductors of said line for inductively and capacitively coupling said device to current-responsive devices, means including said inductive and capacitive coupling for deriving substantially in-phase currents proportional to the current and the voltage respectively in said line, means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said line, means for applying said in-phase currents to one of said current-responsive devices, means for applying said oppositely phased currents to the other of said current-responsive devices, and means connected to said current-responsive devices to indicate the diiference of the squares of the currents in each of 'said currentresponsive devices.

7. A radio frequency wattmeter for a balanced i ductively and capacitively coupling each of said for deriving substantially devices to one of said conductors, a pair of current-responsive devices, means including said inductive and capacitive coupling for deriving sub'- stantially in-phase currents from each of said conductors proportional to the current and voltage respectively in each of said conductors. means including said inductive and capacitive coupling for deriving substantially oppositely phased curto the current and voltage respectively in each of said conductors, means for combining said infor a balanced including said inductive and capacitive coupling for deriving substantially oppositely phased currents from each of said conductors proportional to the current and voltage respectively in each of said conductors, means for combining said inphase currents, means for combining said oppositely phased currents, means for applying said combined in-phase currents to one of said current-responsive devices, means for applying said combined oppositely phased currents to the other of said current-responsive devices, and means connected to said current-responsive devices to indicate the difference of the squares of the currents in each of said current-responsive devices.

9. A radio frequency wattmeter for a balanced transmission line including a coupling device for each conductor ofA said line comprising separate cylindrical conducting members each surrounding and substantially coaxial with a vportion of different conductors of said line for inductively and capacitively coupling each of said devices to one of said conductors, a pair of current-responsive devices, means including said inductive and capacitive coupling for deriving substantially inphase currents from each of said conductors proportional to the current and voltage respectively in each of said conductors, means including said inductive and capacitive coupling for deriving substantially oppositely phased currents from each of said conductors proportional to the current and voltage respectively in each of said conductors, means for combining said in-phase currents, means for combining said oppositely phased currents, means for applying said combined inphase currents to one of said current-responsive devices, means for applying said combined oppositely phased currents to the other of said current-responsive devices, an indicator, and means connecting said indicator to said current-responsive devices.

10. A radio frequency wattmeter for a coaxial transmission line including a coupling device interposed between the inner and outer conductors of said line forinductively and capacitively cou- I 'pling said device to the inner conductor of said line, a pair of current-responsive devices, means including said inductive and capacitive coupling for deriving substantially in-phase currents proportional to the current and voltage respectively in the inner conductor of said line, means including said inductive `and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said inner conductor, means connecting one of said current-responsive devices between said coupling device and said outer conductor of said line to apply said in-phase currents to said current-responsive device, means connecting the other of said current-responsive devices between said coupling device and said outer conductor to apply said oppositely phase currents to said other current-responsive device, an indicator, and means connecting said indicator to said current-responsive devices.

11. A radio frequency wattmeter for a coaxial transmission line including a coupling device interposed between the inner and outer conductors of said line for inductively and capacitively coupling said device to the inner conductor of said line, a pair of current-responsive devices, means including said inductive and capacitive coupling vfor derivirfgsubstantially in-phase currentsgproportional to the current and voltage respectively in the inner conductor of said line,.means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportionalto the current and voltage respectively in said inner conductor, means connecting one of said current-responsive devices between said coupling device and said outer conductor of said line to apply said irl-phase currents to said current-responsive device, means connecting the other of said current-responsive devices between .said coupling device and said outer conductor to apply said oppositely phased currents to said other current-responsive device, and means connected to said current-responsive devices to indicate the difierenceof the squares of the currents in each of said current-responsive devices.

12. A radio frequency wattmeter for a coaxial transmission line including a coaxial cylindrical coupling device of predetermined length interposed between the inner and outer conductors of said line for inductively and capacitively coupling said device to the inner conductor of said line, a pair of current-responsive devices, means including said inductive and capacitive coupling for deriving substantially in-phase currentsproportional to the current and voltage respectively in the inner conductor of said line, means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said inner conductor, means connecting one of said current-responsive devices between said coupling device and said outer conductonof said line to apply said in-phase currents to said current-responsive device, means connecting the other of said current-responsive devices between said coupling device and said outer conductor to apply said oppositely phased currents to said other current-responsive device, an indicator, and means connecting said indicator to said currentresponsive devices.

13. Apparatus of the type described in claim 12 including shielding means for said indicator and said indicator connecting means.

14. A radio frequency wattmeter for a coaxial transmission line including a coupling device interposed between the inner and outer conductors of said line for inductively and capacitively coupling said device to the inner conductor of said line, a pair of current-responsive devices disposed between said coupling device and the outer conductor of said line, means including said inductive and capacitive coupling for deriving substantially in-phase currents proportional to the current and voltage respectively in the inner conductor of said line, means including said inductive and capacitive coupling for deriving substantially oppositely phased currents proportional to the current and voltage respectively in said inner conductor, means connecting one of said currentresponsive devices between said coupling device and said outer conductor of said line to apply said in-phase currents to said current-responsive device, means connecting the other of said current-responsive devices between said coupling device and said outer conductor to apply said oppositely phased currents to said other currentresponsive device, an indicator, and means connecting said indicator to said current-responsive devices.

CHARLES A. ROSENCRANS. 

